Thermoelectric generation indicates the general technique to convert waste heat produced from everyday life and from a variety of industrial fields into electromotive force by using thermoelectric module. That is, it is the technique to convert thermal energy into electric energy by using Seebeck effect. Energy conversion efficiency of the said thermoelectric module depends on figure of merit (ZT) of thermoelectric material. Figure of merit of thermoelectric material, which is in other words thermoelectric figure of merit, is in proportion to temperature (T), and can be determined by Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (κ) of each thermoelectric material (Mathematical Formula 1).ZT=α2σT/κ  (Mathematical Formula 1)(In the Mathematical Formula 1, ZT is the thermoelectric figure of merit, α is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity.)
According to the Mathematical Formula 1, a substance having high electrical conductivity but low thermal conductivity is required to yield high thermoelectric figure of merit. In general, the smaller the size of a particle is, the lower the thermal conductivity goes. The smaller the number of crystal particle through which electric current flows, the higher the electrical conductivity goes. That is, thermoelectric figure of merit can be improved by regulating the crystal growth.
For example, Korean Patent Publication No. 2000-0025229, No. 10-2007-0117270, and No. 10-2010-0053359 describe methods for preparing thermoelectric materials in bulk with improved thermoelectric properties by mechanical milling-mixing. More precisely in those methods, the starting materials Bi and Te were dissolved and hardened, followed by pulverization to give Bi2Te3 elemental powder. Then, the elemental powder proceeded to mechanical grinding to give thermoelectric materials. However, the thermoelectric materials prepared by the above methods have the disadvantage of high thermal conductivity resulted from the particle size in tens of micro-meters.
Korean Patent Publication No. 10-2005-0121189 presents a method for preparing (BiSb)(TeSe) thermoelectric material by melt-spinning and extruding. Particularly, Sb and Se were inserted in BiTe thermoelectric materials to prepare the thermoelectric materials having p-/n-type characteristics. However, the thermoelectric materials prepared by the above method also have the disadvantage of high thermal conductivity owing to the particle size in tens of micro-meters.
In the meantime, Japanese Patent Publication No. 22093024 presents a method for preparing BiTe alloy nanoparticles, in which the Bi precursor BiCl3 and Te were dispersed and reduced in water and then reacted to give Bi2Te3 nanoparticles. However, even though the Bi2Te3 nanoparticles presented in the patent were confirmed to have low thermal conductivity, dispersing agents or reducing agents used in the process were acting as impurities or oxide second phase was produced. Besides, the product is the binary material, which means it does not have enough extrinsic semiconductor properties, so that it is difficult to apply the product to thermoelectric module.
Korean Patent Publication No. 10-2007-0108853 presents a method for preparing nanocomposite designed to decrease thermal conductivity. Particularly, Si nanoparticles having thermoelectric properties were included in Ge host (inclusion), by which electrical conductivity of the complex was kept but thermal conductivity was lowered. The said Si particles are in the size of tens of nano-meters. So, this particles have lower lattice thermal conductivity reduced by phonon comparatively decreased, compared with other particles in the size of tens of micro-meters. However, this thermoelectric composite demonstrates the highest thermoelectric figure of merit in mid-temperature range (around 600 K), and has the disadvantage of requiring high priced raw materials such as Si and Ge.
Korean Patent No. 10-0663975 also describes a method for preparing Fe-doped skutterudite thermoelectric material. Particularly, this method is to reduce thermal conductivity of a thermoelectric material by taking advantage of rattling effect of a specific atom, in which lattice thermal conductivity of each material, crystalline skutterudite and clathrate, can be reduced by inserting rare earth metals and alkali metals in the internal void of the said materials having high electrical conductivity. However, the said skutterudite and clathrate demonstrate the highest thermoelectric figure of merit in mid-high temperature range (higher than 600 K), suggesting that high-temperature/high-pressure process is required.
Lastly, Korean Patent Publication No. 10-2013-0036638 presents a method for synthesizing Bi2TeySe3-y nanoparticles. Precisely in the invention, a [Bi(III)-stabilizer] complex was prepared by using a Bi precursor, a stabilizer, and a reducing agent, which was reacted with anion precursors (Te/Se) to give Bi2TeySe3-y nanoparticles. However, this method has the disadvantage that the said Bi2TeySe3-y nanoparticles need the additional heat-treatment process to eliminate chemical additives used for the preparation, indicating that the particles are further growing because of the heat-treatment and thereby thermal conductivity is increased.
In the course of study to develop a new method for synthesizing a Bi2TeySe3-y nanocompound with improved thermoelectric properties, the present inventors confirmed that a Bi2TeySe3-y nanocompound could be synthesized by liquid phase reduction and this method did not require additional heat treatment to eliminate the added chemical additives with preventing nanoparticles from growing, that is even nanoparticles could be formed. The present inventors further confirmed that the synthesized Bi2TeySe3-y nanocompound had reduced thermal conductivity. Based on the above confirmation, the present inventors developed a method for synthesizing a Bi2TeySe3-y thermoelectric nanocompound with improved thermoelectric figure of merit, leading to the completion of this invention.